Heres how to do it:
You need a credit card in Google wallet, with a US address. I just used my kiwibank visa debit card with a fake US address (added this through the google wallet website).
I then did a factory reset on the tablet, and when it asked me to connect to wifi, I connected to my home wifi, and set a US proxy. I then clicked through the wizard and had a new page saying I got $25 credit. Also using the US proxy means you will get the Play Books, Magazines and Movies app on the tablet (although once you turn off the proxy those won't work anymore).
Also, I think it may be possible to get it without doing a factory reset. In one of my attempts I had done a factory reset, but hadn't had the US address credit card added. I noticed when I went to the play store and attempted to purchase something, under the payment options there was a new option that said "Add a credit card to get $25". I tried to add some fake credit cards at that step, but they wouldn't work. I'm not sure if that option will show up once you are connected to a US proxy or not.